I don't think you'll face any timing problems with 8259. Since it is a static device working without any clock or polling, as soon as it receives any interrupt, it will send IRQ to the uC with a time delay equivalent to its propogation delay (few nano Sec.).
The real time delay will occur after this. uC will finish its current instruction and then go to the ISR which will take a few integral number of clocks. But in the mean time if other interrupt occurs, 8259 will also handle it. So for better speed of response use higher crystal speed.